This invention relates generally to micromachining methods. More particularly, it relates to a method for producing sub-millimeter parts by Micro Electric-Discharge Machining using electrodes fabricated by silicon-processing techniques.
In the past few decades, techniques have been developed for manufacturing sub-millimeter sized parts out of a limited set of materials. Generally, micromachining techniques can produce parts with resolutions of a few micrometers or better. For these techniques to be economically feasible, they must be adaptable to parallel manufacturing, in which many of the same or different parts are produced in one step, and allow a simple transition from a geometric computer-aided design (CAD) model to the physical part.
LIGA (Lithographie, Galvanik, Abformung) is a micromachining technique that creates highly anisotropic patterns in polymeric material by exposing it to monochromatic X-ray radiation through masks. Irradiated regions are easily removed to produce polymer structures with high aspect ratios. Polymer structures are then filled with ceramics or electroplated metals to generate desired microstructures. While LIGA produces high-precision parts, it is limited to part materials that can easily be deposited in the polymer structure. Additionally, electroplating allows no control over the microstructure of the resulting metal part, thereby limiting strength and magnetic properties. Further, the monochromatic X-rays needed are produced in synchrotrons, which are enormously expensive to operate, and of which there is only a handful throughout the world. Patterning a single 3-inch polymer disk takes an entire day. LIGA is a useful research tool but is not feasible for economic mass production.
Silicon and thin film processing can be used to create structures from silicon and compatible materials. Materials are deposited on a silicon support by sputtering or electroplating and shaped by etching. These techniques, while much less expensive than LIGA, also have significant limitations. Not all materials can be deposited in the necessary quality using thin-film techniques. Feature height of sputtered parts is limited by internal stress that causes delamination above 10 xcexcm. There is also no way to control crystalline structure of the material being deposited, which substantially limits potential applications of the microstructures.
In fact, most existing micromachining techniques are significantly limited in the material that can be machined, and are best suited to producing parts, or sub-parts that can be combined to form an assembled part, with heights below 100 xcexcm. Conventional machining techniques generally have a minimum part size (height or width) of 1 mm. Parts between 100 xcexcm and 1 mm, the mesoscale range, are difficult to manufacture using micro or conventional machining techniques.
Electric-Discharge Machining (EDM) is a macroscale technique for fabricating large parts from conductive materials that are too hard to be machined with conventional techniques such as milling or turning. Controlled, short-duration electric discharges are generated between a patterned electrode and a conductive workpiece to remove material by electroerosion. The electrode also wears away during machining, and must be replaced as necessary. Conventional EDM can produce structures or features larger than 1 mm.
EDM has been modified for use in micton-sized applications using different types of electrodes. Wire Electro-Discharge Grinding (WEDG) uses a thin wire electrode. The patterned part is created by moving the workpiece with respect to the wire, thereby xe2x80x9ccuttingxe2x80x9d the workpiece into the correct shape. These and similar processes that use rotating disks as electrodes are described in xe2x80x9cMicro Electro-Discharge Machining as a Technology in Micromachining,xe2x80x9d W. Ehrfeld et al., Proceedings of the SPIE, Volume 2879, pp. 332-337 (1996). Because each part must be carefully cut into the desired pattern, and not simply transferred from a patterned electrode as with conventional EDM, existing micro-EDM is a very slow and tedious technique.
A process for using LIGA to create patterned micro-electrodes for EDM is described in xe2x80x9cA Novel Micro Electro-Discharge Machining Method Using Electrodes Fabricated by the LIGA Process,xe2x80x9d K. Takahata et al., 12th IEEE International Conference on Micro Electro Mechanical Systems, Technical Digest, pp. 238-243 (1999). Because micro-electrodes created by LIGA contain the entire part pattern (its inverse, actually), they reduce processing time significantly over wire and rotating disk micro-electrodes. Currently, LIGA electrodes can be produced only by electroplating. Because not all metals can be electroplated easily, only a few electrode materials can be used. For example, tungsten is commonly used as electrode material in WEDG and conventional EDM, because its high melting temperature (3400xc2x0 C.) provides excellent wear resistance. Tungsten cannot be electroplated easily, and its processing temperature is much higher than polymers can withstand; it is not a viable material for LIGA-produced electrodes. More importantly, LIGA is prohibitively expensive and highly inaccessible, and simply cannot be used for mass production. Because electrodes wear down during the EDM process, new electrodes must be continually made. Electrode fabrication by LIGA cannot be incorporated into an integrated manufacturing system.
There is still a need for an economically viable micro-EDM method that uses a variety of easily-produced patterned electrodes.
Accordingly, it is a primary object of the present invention to provide a microfabrication technique for parts made of any type of conductive material, including pure metals, alloys, and magnetic materials.
It is a further object of the invention to provide a technique for fabricating parts of up to 1 mm in height.
It is an additional object of the invention to provide a method that does not alter crystal structure of the part material, preserving macroscopic strength, hardness, and magnetic properties.
It is another object of the present invention to provide a variety of techniques for producing micro-EDM electrodes from a range of materials.
It is another object of the invention to provide a cost-effective micromachining technique that can be used in commercial, mass-production applications.
It is a further object to provide a technique that allows for parallel production methods, and can be used in an integrated CAD/CAM system.
These objects and advantages are attained by a method of fabricating arbitrarily shaped microstructures from a conductive workpiece by silicon Electric-Discharge Machining (EDM). A small and precise pattern is first created on a silicon wafer, and then the pattern is transferred onto the desired material. The process produces microparts of up to one millimeter in height with a resolution of one micrometer. Crystalline structure of the workpiece is preserved in the fabricated micropart, and the method is suitable for shaping hard and soft magnetic materials, single crystals, and special alloys.
In a first step, a patterned metal or silicon EDM electrode is produced using standard silicon-processing techniques. The pattern contains features with heights preferably between 1 xcexcm and 1 mm. During the EDM process, an electric discharge is generated between the electrode and a conductive workpiece to remove workpiece material corresponding to the pattern. The remaining workpiece is the micropart. The workpiece can be an amorphous metal, a crystalline metal, a magnetic material, or any other conductive material. The magnetic material may consist of regions of different magnetic orientations, produced by adhering two different materials together. The electrode can then be placed directly over the boundary between the two materials to form the part.
In a first method for creating the electrode, a silicon wafer is etched to produce a surface pattern. A metallic layer is then deposited on the pattern, preferably by sputtering, to a predetermined thickness, such that the metallic layer conforms to the pattern. The silicon wafer with the metallic layer is the EDM electrode; the discharge is generated between the workpiece and the metallic layer. The metallic layer is used to increase the electrical conductivity of the electrode surface. In this method, the pattern on the silicon wafer is an inverse of the micropart pattern.
In a second method, a silicon wafer is again etched to produce a surface pattern. The pattern is completely filled with a metal to create an inverse pattern in the metal. The silicon wafer is then removed from the metal, preferably by chemical etching, and the metal is used as an EDM electrode. For this type of electrode, the pattern on the silicon wafer is the same as the pattern on the micropart, and the electrode pattern is an inverse of both of these patterns.
The patterned silicon wafer can be filled with metal in one of two ways. The metal, preferably copper or nickel, can be electroplated into the wafer. Alternately, a metal powder, preferably a mixture of silver and tungsten, can be hot-pressed into the wafer to form a solid electrode.
The method of the present invention can be fully automated for manufacturing purposes, and an integrated CAD/CAM (computer-aided manufacturing) system can produce finished parts directly from a three-dimensional CAD model.